1. Field of the Invention
The present invention relates to a memory and operation method thereof; in particular, to a phase change memory (PCM) and operation method of the same.
2. Description of Related Art
Phase change based memory materials are widely used in read-write optical disks. These materials have at least two solid phases, including for example a generally amorphous solid phase and a generally crystalline solid phase. Laser pulses are used in read-write optical disks to switch between phases and to read the optical properties of the material after the phase change.
Phase change based memory materials, like chalcogenide based materials and similar materials, also can be caused to change phase by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access.
In detail, a PCM can be programmed and reprogrammed into high and low resistance states. When a phase change material is in the amorphous phase (known as the RESET state) it has high resistance; but when it is in the crystalline phase, it has low resistance (known as the set state). FIG. 1 shows the curves of voltage versus time for a RESET pulse and for a SET pulse in the PCM. As shown in FIG. 1, to ensure the “SET” programming in which the phase change material in the PCM will crystallize, the temperature is generally higher than the crystallization temperature and be held for a long time (t) to complete the process of SET the PCM. In other words, a long pulse should be used in the “SET program”.
On the other hand, as shown in FIG. 1, for a “RESET program” in which the phase change material in the PCM is changed from a SET state to a RESET state, a higher voltage is applied into the phase change material for heating it above the melting temperature (Tm). In order to melt the phase change material and rapid quench down, a shorter and high-V pulse is used in the “RESET program”.
However, for SET, a longer time is need to crystal the phase change material as shown in FIG. 1. Therefore, the write speed of the PCM will be lowered by the long SET time.
In view of the foregoing, there is a need for a method for accelerating the SET process in the PCM.